The present invention relates to a process for igniting a sinter mix composed of a solid fuel and a sintering material, in particular an ore reduction sintering mix on a sintering machine in which the sintering mix is passed underneath an igniting kiln having closed end and side walls and a closed top, wherein in the igniting kiln hot flue gases are generated above the sintering material and these hot flue gases heat up and ignite the surface of the sintering material by radiation and convection. The invention furthermore relates to an apparatus for carrying out such a process with an igniting kiln which is open in the downward direction with two end walls, two side walls and a top and with a sintering belt for carrying the sintering mix, movable substantially horizontally therebelow in the direction of the connecting line between the end walls, the end walls and the side walls extending downwardly into close proximity to the sintering mix so that a hood-like igniting kiln space substantially closed off from the outer atmosphere is formed.
Igniting kilns for igniting sinter mixes are frequently designed as hoods which are closed towards the top and along the sides, whilst being downwardly open. Underneath such igniting kilns the sintering mix is conveyed through in a layer thickness of approximately 40 cm. on a so-called sinter belt which conventionally is composed of an endless series of fire grid carriages in direct abutment with one another. The sintering mix, for example for steel production, is composed substantially of iron ore serving as the sintering material and coke as the solid fuel, as well as some additives which depend upon the particular steel-making process.
In order to ignite the sintering mix whilst passing underneath the igniting kiln, this is fitted with burners which generate the temperatures required for ignition. Suction shafts are provided underneath the sintering belt by means of which the combustion gases are drawn from the igniting kiln through the sintering mix.
For the economical production of a sinter which is adapted to the characteristics of the subsequent smelting process, it is important, with regard to the ignition, that the ignition takes place on the surface of the sintering material intensively and rapidly, as well as uniformly with regard to the transverse direction to the direction of conveyance. This is to be attained with the smallest possible fuel input, not only with regard to the solid fuel in the sintering mix but also with regard to the fuel for the burners in the igniting kiln which is usually gaseous or liquid. Finally, the economics of the process are substantially influenced by the possible throughput capacity of the plant which, in turn, is decisively dependent upon the quality and velocity of the ignition procedure.
Igniting kilns of the initially described type are already known in various forms of construction. For example, there are igniting kilns in which the burners are fitted in the roof or in the end walls downwardly inclined, the burner jets of the individual burners being aimed on to the surface of the sintering material. This process admittedly results in a strong heating of the surface of the sintering material but gives a non-uniform ignition because those parts of the sintering material surface which lie in the middle point of the particular burner jet are thereby heated more intensely than the regions lying between the burner jets. A modification of this mode of construction consists in that the burners are arranged in the end walls of the igniting kiln, aimed towards each other with an oblique downward inclination. This results in a flow directed towards the roof in the centre of the igniting kiln where the flue gases of the burners meet one another, by which the hot particles of sintering materials are entrained upwardly and thus result in progressively growing deposits on the roof of the kiln.
In order to overcome these disadvantages and to attain an improved ignition, a construction has already been proposed in which the burners are arranged in the two side walls of the igniting kiln and substantially horizontally. This solution thus avoids the direction of the burner jets on to the surface of the sinter bed. Heating and ignition of the surface of the sinter bed thereby takes place more by the radiation of the kiln space of the igniting kiln. However, the influence of the non-uniform temperature distribution over the long range of the individual burner jets is the same for all burners arranged one behind the other and, accordingly, once again results in different temperatures over the cross-section of the sinter material surface. Furthermore, in this form of construction, the burner jets in the case of the small width of the sinter kilns of about 2 to 5 meters, already meet each other after 1 to 2.5 meters, which implies the risk of incomplete combustion and of stirring up the sinter bed in the centre of the kiln.
It has also been described (Fred. Cappel and Alois Kilian: "Zundung von Sintermischungen", Stahl und Eisen, 94 (1974) No. 11, page 453) to elongate the igniting kiln proper and to have this followed by a so-called thermal treatment part. In this case, the burners of the inlet portion of the elongated igniting kiln are operated with an approximately stoichiometrical air ratio, whilst the burners of the outlet side, i.e. the thermal treatment part, are operated with a relatively large excess of air. As a result, in the thermal treatment part, the oxygen required for the reaction with the solid fuel is introduced into the sinter bed in a heated state, whereby the ignition throughout is improved.
In this process, maximum possible temperature for a given fuel input is attained in the section on the inlet side due to the stoichiometrical mode of operation of the burners. The oxygen required for the combustion is first introduced into the thermal treatment part in that the burners are there operated with a comparatively large excess of air. As a result, as has been recognised by the present invention, the heat generated by the burners on the inlet side is only partly utilised so that an unnecessarily high energy consumption results.